The present invention pertains generally to a device for detecting a fault in a solenoid valve (electromagnetic valve), and particularly to a technique for detecting a fault in a solenoid valve employed for an electronic brake control system of a vehicle.
For example, Japanese Patent Application No. 8-250702 discloses a technique relating to an electronic brake control system of a vehicle. According to the electronic brake control system, hydraulic pressure of a wheel cylinder is controlled by using a hydraulic valve (solenoid valve), whose actuation is controlled by means of adjusting a current introduced into a solenoid valve coil by a controller comprising an electronic circuit. Since such a solenoid valve is used for a vehicle wherein safety is critically important, a fault occurring in such a solenoid valve must be detected promptly.
For example, one such device for detecting a fault in a solenoid valve has a circuit configuration as indicated in FIG. 1. In the circuit, the numeral 1 denotes a battery of an automobile; 2 is a relay; 3 is a solenoid valve coil; 4 is a switching FET (Field Effect Transistor); 5 is a relay drive circuit for driving the relay 2; 6 is a FET drive circuit for driving the switching FET 4; 7 is a microcomputer (detecting means); 8 is a resistance-type potential divider; 8a and 8b are resistors, each having a high resistance which comprise a resistance-type potential divider 8; 9 is a battery power supply line (wiring); 10 is an electric power supply line of a solenoid valve coil connecting line (wiring); 11 is a grounding line of the solenoid valve coil connecting line (wiring); P1 and P2 are digital output ports of the microcomputer 7; and P3 is a digital input port of the microcomputer 7.
A negative terminal of the battery 1 is grounded, whereas a positive terminal is coupled to one end of the relay 2 via the battery power supply line 9. The other end of the relay 2 is connected to one end of the solenoid valve coil 3 via the electric power supply line 10, whereby, for example:, a drain terminal of the switching FET 4 is connected to another end of the solenoid valve coil 3 via the grounding line 11 and a source terminal of the switching FET 4 is grounded. In other words, the relay 2 is connected to the electric power supply line 10 side of the solenoid valve coil 3 and the switching FET 4 is connected to the grounding line 11 side of the solenoid valve coil 3, whereby a serially-connected circuit consisting of the relay 2, solenoid valve coil 3 and switching FET 4 is connected to both ends of the battery 1.
Further, the relay drive circuit 5 is driven by an output signal from the digital output port P1, while the FET drive circuit 6 is driven by an output signal from the output port P2. The grounding line 11 is connected to the digital input port P3 via the resistance-type potential divider 8. In other words, a voltage of the grounding line 11 is divided by the resistors 8a and 8b, each having a high resistance to be inputted into the digital input port P3. The battery power supply line 9, relay 2, electric power supply line 10, grounding line 11 and switching FET 4 constitute a feeding system for supplying a drive current to the solenoid valve coil 3.
According to a solenoid valve fault detecting device having the above-described constitution, the digital output ports P1 and P2 output signals to turn on/off the relay 2 and/or switching FET 4, and a signal inputted to the digital input port P3 at this time is to detect a fault in the solenoid valve coil 3 and the foregoing feeding system.
For example, if the relay 2 and switching FET 4 are turned off by the output signals from the digital output ports P1 and P2 of the microcomputer 7 while the battery power supply line 9, relay 2, electric power supply line 10 of the solenoid valve coil 3, solenoid valve coil 3, grounding line 11 of the solenoid valve coil 3 and switching FET 4 are operating normally, a current does not run in the solenoid valve coil 3 and thus, a "L" (low) level input signal is supplied to the digital input port P3 of the microcomputer 7.
Further, when the relay 2 is turned on by the output signal from the digital output port P1 of the microcomputer 7 and the switching FET 4 is turned off by the output signal from the digital output port P2 of the microcomputer 7, an electric current from the battery 1 runs in a series circuit consisting of the relay 2, solenoid valve coil 3 and resistors 8a and 8b. An electric current sufficient to drive the solenoid valve does not run in the solenoid valve coil 3 because the resistors 8a and 8b have a high resistance, and a "H" (high) level signal is inputted to the digital input port P3 of the microcomputer 7 via the resistance-type potential divider 8 consisting of the resistors 8a and 8b.
Still further, when the relay 2 and the switching FET 4 are both turned on by the output signals from the digital output ports P1 and P2 of the microcomputer 7, respectively, an electric current sufficient to drive the solenoid valve 3 runs in the solenoid valve coil 3 and thus, the level of the input to the digital input port P3 of the microcomputer 7 becomes "L" (low).
If a fault occurs in the battery power supply line 9, electric power supply line 10 of the solenoid valve coil 3, grounding line 11 of the solenoid valve coil 3, relay 2 or switching FET 4, the following state occurs.
For example, when the electric power supply line 10 or the grounding line 11 are short circuited with the battery power supply line 9, a "H" (high) level signal is inputted to the digital input port P3 of the microcomputer 7 despite the fact that the microcomputer 7 is outputting, from the digital output ports P1 and P2, signals for turning off the relay 2 and the switching FET 4.
Further, when the electric power supply line 10 the grounding line 11 is disconnected or grounded, when the battery power supply line 9 is disconnected or when the relay 2 cannot be turned on due to a problem in the relay drive circuit 5 or the relay 2, a "L" (low) level signal is inputted to the digital input port P3 of the microcomputer 7 despite the fact that the microcomputer 7 is outputting from the digital output port P1 a signal for turning on the relay 2 and from the digital output port P2 a signal for turning off the switching FET 4.
When the switching FET 4 cannot be turned on due to a failure in the FET drive circuit 6 or the switching FET 4, a "H" (high) level signal is inputted to the digital input port P3 of the microcomputer 7 despite the fact that the microcomputer 7 is outputting from the digital output ports P1 and P2 signals for turning on the relay 2 and the switching FET 4, respectively.
When a fault occurs in the feeding system for supplying the current to the solenoid valve coil 3, the aforementioned prior art device detects such a fault by checking a voltage level at the digital input port P3 in various operating states of the relay 2 and the switching FET 4 as is described above. For example, when such a fault as grounding or disconnection of the electric power supply line 10 or the grounding line 11 occurs, the digital output port P1 of the microcomputer 7 outputs a signal for turning on the relay 2, and the microcomputer 7 checks whether or not the voltage level at the digital input port P3 becomes "H" (high) when the relay 2 is turned on, thereby determining whether the fault occurred.
According to the foregoing prior art device, grounding or disconnecting the electric power supply line 10 or the grounding line 11 is detected when the relay 2 is on and thus, a heavy current from the battery 1 runs in the battery power supply line 9 and the relay 2 and can damage them when, for example, such a fault as grounding of the electric power supply line 10 occurs. Further, since the solenoid valve coil 3 has low resistance and the resistors 8a and 8b comprising the resistance-type potential divider 8 have a high resistance, a voltage level at the digital input port P3 of the microcomputer 7 in the case where both ends of the solenoid valve coil 3 are short-circuited with the relay 2 being on, indicates a very small change from the voltage level in the case where the solenoid valve coil 3 is operating normally. Thus, the prior art device is not able to detect short circuiting, etc., of the solenoid valve coil 3, which is disadvantageous.